1. Field of the Invention
This invention relates to a photographing system, such as a camera, and, in particular, to a flash photographing system including a camera unit and a flash unit.
2. Description of the Prior Art
A flash photographing system typically includes a camera unit and a flash unit. In order to carry out a flash photographing operation properly, it is necessary that a plurality of information signals can be communicated between the camera and flash units. One example of this will be described with reference to FIG. 12. As shown, in a typical prior art flash photographing system, the camera unit 1 is connected to the flash unit 2 through three kinds of signal lines l.sub.1 -l.sub.3 by means of a hot shoe, a connection code, etc. The camera unit 1 is typically provided with first through third transmission terminals la, lb and lc, and the flash unit 2 is similarly provided with corresponding first through third transmission terminals 2a, 2b and 2c. Each of the three signal lines l.sub.1 -l.sub.3 extends between the corresponding pair of transmission terminals between the camera unit 1 and the flash unit 2.
For example, the first signal line l.sub.1 extending between the first transmission terminal 1a of the camera unit 1 and the first transmission terminal 2a of the flash unit 2 is provided to carry a flash start signal X from the camera unit 1 to the flash unit 2. The pair of these first transmission terminals 1a and 2a is typically implemented in the form of a hot shoe which is fabricated according to industrial standards, such as Japanese Industrial Standards. A plurality of information signals A originating from the flash unit 2 are serially supplied from the flash unit 2 to the camera unit 1 through the second signal line l.sub.2 extending from the second transmission terminal 2b of the flash unit 2 to the second transmission terminal 1b of the camera unit 1. In addition, a timing signal B is supplied from the camera unit 1 to the flash unit 2 through the third signal line l.sub.3 extending from the third transmission terminal 1c of the camera unit 1 to the third transmission terminal 2c of the flash unit 2. The timing signal B is used to select the plurality of information signals A originating serially from the flash unit 2.
Typically, the flash start signal X is produced at the time when the shutter (not shown) provided in the camera unit 1 has been set fully open, and when this flash start signal X is supplied, the flash unit 2 initiates its flashing operation. The flash unit originating information signal A includes a maximum guide number possessed by the flash unit 2, a flash stop signal, an automatic light adjustment completion signal, a charging completion signal, etc. which are serially supplied from the flash unit 2 to the camera unit 1. The timing signal B is used to make selections for the various signals serially supplied from the flash unit 2 to the camera unit 1, so that, based on the timing signal B, desired signals from the flash unit 1 are supplied to the camera unit 1 as the flash unit originating information signal A. For further information in this respect, reference should be made to Japanese Patent Laid-open Publications, Nos. 59-189332, 59-191022 and 59-191023, which are hereby incorporated by reference.
Accordingly, various information signals are transmitted from the flash unit 2 to the camera unit 1 using the second and third transmission terminals 1b-2b and 1c-2c other than the first transmission terminals 1a and 2a, which are used to transmit the flash start signal X, so that the structure shown in FIG. 12 allows to carry out a flash photographing operation efficiently with the use of a minimum number of signal lines. However, in such a typical prior art flash photographing system, information to be supplied from the camera unit 1 to the flash unit 2, such as a set stop value in the case of an automatic flashing operation or automatic/manual setting information, cannot be transmitted. It is true that such information can be transmitted by providing additional signal lines and transmission terminals; however, the provision of such additional transmission terminals tends to make the entire system complicated in structure. In particular, in the case of the hot shoe type, the electrical connection between the camera unit 1 and the flash unit 2 tends to become unreliable, and attaching the flash unit 2 to the camera unit 1 becomes difficult. Besides, the exchangeability with other common flash units will be lost.